In recent years, along with progress of the semiconductor technology, digitalization of information has been progressed rapidly and, along with digitalization of information, particularly, digitalization of image information such as static or moving picture, the amount of information to be treated has become enormous. In such a situation, it is necessary to store the information temporarily or quasi-permanently. As means for recording the enormous information, optical recording media such as optical discs are considered to be promising and research and development have been conducted vigorously for increasing the capacity.
Particularly, a so-called phase-change optical recording medium, which records and erases information by utilizing a material that conducts reversible phase change between two states, namely, a crystalline state and an amorphous state, is highly expected as an image recording medium, since it has an excellent feature capable of recording and erasing information by a simple optical system and easily so-called overwriting, which conducts recording new information while erasing already recorded information simultaneously.
As a recording material for the phase-change recording medium, chalcogenide alloys such as Ge--Te--Sb series alloys (refer to Japanese Patent Laid-Open Sho 62-53886 and Japanese Patent Laid-Open Sho 61-258787), In--Sb--Te alloys (refer to Japanese Patent Laid-Open Sho 62-241145) or oxide series materials such as Te--Ge--Sn--O have been used mainly. Upon overwriting, a portion of a recording layer irradiated by a laser beam with a recording power is made amorphous by being heated to higher than the melting point and then quenched to form a recording mark, whereas a portion of the recording layer irradiated by a laser beam with an erasing power is crystallized to erase the recording mark by being heated to higher than the crystallizing temperature.
When recording and erasing of information are actually conducted by using the recording material described above, a protection layers are usually located just beneath and just above the recording layer for preventing the recording layer from oxidation or deformation. Further, a structure of disposing a reflection layer through a protection layer on a recording layer is often used as a more preferred layer structure for picking up an optical change in the recording layer as a great change of reflectance by utilizing the interference effect of light.
As the material for the protection layer, oxides, carbides, nitrides, flouorides and sulfides of metals or semimetals, and the like have been known and, among them, ZnS has been known to provide a strong adhesion with the recording layer. However, a protection layer consisting only of ZnS was insufficient for the heat resistance since crystalline grains of ZnS becomes coarser by repetitive recording and erasing by overwriting. Japanese Patent Laid-Open Sho 63-103453 discloses an optical disc enhanced with the heat resistance of the protection layer and improved with respect to deterioration of cyclability caused by thermal deformation or the like of the recording layer, by the addition of a glass-forming material such as Sio.sub.2 to ZnS. Such existent material for the protection layer as described above is usually transparent to a light used for recording, erasing and reading.
On the other hand, as a method of increasing the recording density, in a case of existent mark position recording of corresponding a recording signal to a central position of a recording mark, there is a method of increasing the density in the direction of a recording track (linear density) and the density in the direction vertical to the recording track (track density) by reducing the size of recording marks so as to bring the spaced recording marks closer to each other.
Further, mark edge recording of corresponding a recording signal to the position for the leading end and the trailing end of a recording mark can increase the recording density in the track direction by about 1.5 times compared with the mark position recording for the recording mark of an identical size, so that the recording density can be increased by conducting the mark edge recording instead of the mark position recording.
In the case of increasing the density in the mark position recording, it is necessary to shorten the wavelength of a light source for recording in order to greatly reduce the size of the recording mark, whereas the mark edge recording provides a merit capable of increasing the density without substantial alteration of an apparatus. However, in a case of recording information by the mark edge method, it is necessary to form a recording mark precisely with a constant width at a predetermined position.
In such an existent phase-change recording medium, since the absorption factor for the laser beam in a crystalline state is lower than the absorption factor for the laser beam in an amorphous state and, in addition, the heat conductivity is higher and diffusion of heat generated is larger in the crystalline state, a portion in the amorphous state was heated to a higher temperature compared with a portion in the crystalline state even when a laser beam with an identical intensity was irradiated. Namely, there was a phenomenon that the temperature elevation was different depending on the preceding state whether it was in the crystalline state or in the amorphous state even when a laser beam with an identical intensity was irradiated. If the temperature elevation was different as described above, it resulted in a problem that the width of the marks to be formed was not uniform, or the mark forming position was displaced from a normal position.
As a means for solving the foregoing, it is disclosed, for example, in Japanese Patent Laid-Open Hei 1-149238 that the light absorption rate in the crystalline state of the recording layer (hereinafter referred to as [Ac]) is made equal with or higher than the light absorption rate in the amorphous state (hereinafter referred to as [Aa]). For this method, it is mentioned to properly select the thickness for each of the layers constituting the phase-change optical recording medium. Specifically, it is disclosed an example of increasing the ratio (Ac/Aa) between the crystallized state and the amorphous state with respect to the light absorption rate to about 1.1 by reducing the thickness of a metal reflection layer than usual and changing the thickness of the protection layers just above and just beneath the recording layer.
That is, in this method, Aa is kept low by reducing the thickness of the reflection layer to increase the transmitted light therethrough, but increase of the transmittance in the reflection layer involves a problem that each of the performances in recording, erasing and reading is deteriorated since the light returned from the reflection layer as a back surface of the phase-change optical recording medium causes noises upon reading, or cooling performance of the recording layer is lowered due to the reduced thickness of the reflection layer.
Further, the Ge--Te--Sb alloy requires a technique of greatly increasing the ratio (Ac/Aa) of light absorption rate compared with the phase change material of other oxide series. This is considered to be attributable to the change of the values of thermophysical property along with the phase change of the Ge--Te--Sb alloy.
That is, since the property of the Ge--Te--Sb alloy becomes similar as that of metal along with the phase change from the amorphous state to the crystalline state, it is supposed that the change of the thermal conductivity is larger compared with other materials. As a result, it is considered that when a recording mark is formed to a portion in the crystalline state, a higher ratio (Ac/Aa) of light absorption rate is necessary, since a considerably greater amount of heat is necessary compared with a portion in the amorphous state.
In view of the foregoing, in the field of the phase-change optical recording medium, it has been keenly demanded for a designing method for increasing the ratio (Ac/Aa) of light absorption rate in the crystallized state to the light absorption rate in the amorphous state and a technology effective in increasing the ratio (Ac/Aa) of light absorption rate.
In addition, the existent phase-change recording medium involves a problem that each of the performances in the recording, erasing and reading is deteriorated if recording and erasing are repeated by a number of cycles. It is considered that the deterioration of the performance caused by the number of repetitive cycles is due to the following reasons.
Namely, since the intensity distribution of a laser beam has a Gaussian distribution, temperature of elevation and cooling, and change thereof with lapse of time are different between the central portion and the end portion in the area irradiated with a laser beam, and phase separation or segregation of the recording material occurs in the amorphous mark formed. As a result, phase separation or segregation is increased by the repetitive recording and erasing to change the characteristics and shorten the life for repetitive cycles.
Further, when recording and erasing are conducted repetitively, delamination occurs at the interface between the protection layer and the recording layer or the material of the recording layer is fluidized by the stress generated in the protection layer in the course of melting the material of the recording layer during recording to result in local fluctuation of the film thickness, which also leads to deterioration of the life for repetitive cycles.
In order to prevent the degradation of the performance due to repetitive recording and erasing, Japanese Patent Laid-Open Sho 63-217542 discloses that a light absorbing PbS layers is disposed between the recording layer and the protection layer to make the temperature distribution uniform and Japanese Patent Laid-Open Hei 7-262614 discloses that a W (tungsten) layer is disposed between the recording layer and the protection layer to increase the adhesion between them.
However, disposition of the PbS layer or the W layer between the recording layer and the protection layer provides a problem that a difference of the reflectance, namely, an optical contrast between the amorphous state and the crystalline state can not be ensured sufficiently. This problem has not been known so far but is considered to be attributable to a large refractive index of W or PbS.
Furthermore, in the structure of disposing the layer comprising a light absorbing material (light absorption layer) between the recording layer and the protection layer, quenching necessary for making the recording layer amorphous is not conducted due to generation of heat caused by light absorption in the light absorption layer, to result in a worry that no sufficient amorphous state is obtained.
Furthermore, with the reasons as described in (1)-(3) below, in order to increase the recording density, it is necessary to rapidly dissipate the heat generated in the recording layer to the outside of the recording layer after passage of a recording beam.
(1) If the heat generated in the recording layer prevails in the recording layer, since temperature rises within an entire plane of the recording layer, it is difficult to reduce the size of the recording mark. PA1 (2) If the distance between each of the spaced recording marks is decreased, since heat generated upon forming the just preceding mark tends to be transferred as far as the position for forming the just succeeding mark, the position for forming the succeeding mark tends to be displaced from the normal position. PA1 (3) When the heat prevails in the plane of the recording layer, since heat flows from a leading portion of the mark to a trailing mark forming portion, the temperature to be reached is made higher than that at the position for starting the mark formation. As a result, the width at the trailing end of the mark is increased to distort the shape of the mark into a tear drop shape.
As described above, for conducting mark edge recording precisely in a phase-change optical recording medium in which the light absorption layer is disposed between the recording layer and the protection layer (that is, just above and/or just beneath the recording layer), it is necessary to sufficiently diffuse heat from the recording layer and the light absorption layer.
In view of the foregoing, it is a first object of the present invention to increase the ratio (Ac/Aa) of light absorption rate in the crystalline state relative to the amorphous state without deteriorating each of the characteristics in recording, erasing and reading in a phase-change optical recording medium having a light recording layer comprising a Ge--Te--Sb alloy. Further, it is a second object of the present invention to enable to precisely conduct mark edge recording at high recording density in a phase-change optical recording medium having light absorption layer just above/or just beneath the recording layer.